For the fajitas: In a large bowl or resealable plastic bag, combine the steak, onions, green peppers and red peppers with the oil, lime juice, orange juice, coriander, cumin, cayenne, garlic, 2 1/2 teaspoons salt and a few grinds of black pepper. Rub well with your hands to coat. Marinate in the refrigerator for at least 2 hours and up to overnight.
For the guacamole: Mash the avocados with the lime juice in a medium bowl. Stir in the red onion, cilantro and jalapeno and season with salt. If not serving right away, press plastic wrap to seal the surface and prevent browning.
Prepare an outdoor grill or large 2-burner grill pan for medium-high heat. Lightly brush the grill grates with oil.
If necessary, cut the steak into lengths that will fit on your grill. Grill the steak, turning once, until well-marked and cooked to medium-rare, 3 to 5 minutes per side, depending on thickness. Transfer to a cutting board and cover loosely with foil.
Add the onions and peppers (from the marinade) to the grill and cook, turning occasionally, until lightly charred and slightly softened, about 10 minutes. Transfer to a large platter.
Thinly slice the steak against the grain and transfer to the platter with the grilled vegetables.
Serve with the warm tortillas, guacamole, pico de gallo and cilantro.